The present invention relates to a low-expansion unit and in particular to the low-expansion unit that is utilized as a heat spreader for use in a semiconductor apparatus. The present invention also relates to a method of manufacturing such a low-expansion unit and to a semiconductor apparatus provided with the low-expansion unit.
A prior art semiconductor apparatus is shown in FIG. 4. In the prior art structure, an insulated layer 2 is formed on a surface of a substrate 1 made of aluminum. A semiconductor device 4 is joined on a wiring layer, which is formed on a surface of the insulated layer 2 and is not shown, through a solder 3.
The heat generated by the semiconductor device 4 is transmitted to the substrate 1 through the insulated layer 2. Because the substrate 1 is made of aluminum, whose thermal conductivity is relatively superior, the heat is efficiently radiated from the substrate 1 to the outside thereof.
In the above structure, while the semiconducting material such as silicon used in the semiconductor device 4 has a relatively small thermal expansion coefficient, aluminum, which is used in the substrate 1, has a relatively large thermal expansion coefficient. Therefore, thermal stress is generated between the substrate 1 and the semiconductor device 4 as a consequence of a change of temperature. This thermal stress can cause the semiconductor device 4 to warp and the solder 3, which is used for joining the semiconductor device 4 to the insulated layer 2, to crack.
In order to relax the thermal stress, for example, in a semiconductor apparatus for use in a vehicle where temperature differential is extremely large, a heat spreader 5 is installed between the semiconductor device 4 and the insulated layer 2. (See FIG. 5)
As shown in FIG. 6, the heat spreader 5 is a composite material in which at least a copper material 7 sandwiches a plate 6 made of invar on both surfaces of the plate 6. Japanese Unexamined Utility Modal Publication Nos. 63-20448 and 63-20449 disclose such heat spreaders. Invar is an alloy whose thermal expansion coefficient is extremely small. Consequently, the plate 6 is hardly expanded by heat at normal temperature. Therefore, thermal stress is relaxed by installing the semiconductor device 4 on the heat spreader 5.
Still referring to FIG. 6, in the heat spreader 5, the plate 6 made of invar has a relatively low thermal conductivity, about 10 W/mK. Therefore, while the copper material 7 has a relatively superior thermal conductivity, the thermal conductivity in the direction of thickness of the heat spreader 5, which is formed by sandwiching the plate 6 with the copper material 7, becomes relatively low. In this case, the thermal conductivity is about 30 W/mK. Due to the relatively low thermal conductivity, radiating performance of the heat spreader 5 is lowered. Furthermore, the composite material for use in the heat spreader 5 is relatively costly.